Coaxial dual-unit electrodynamic loud-speaker



Sept. 2,1947. PR ST N 2,426,948

COAXIAL DUAL-UNIT ELECTRODYNAMIC LOUD.SPEAKER Filed Jan. 2, 1945 2 Sheets-Sheet 1 IN V EN TOR.

DB .fo/m Peasrorz Patented Sept. 2, 1947 v COAXIAL DUAL-UNIT ELECTRODYNAMIC LOUD-SPEAKER John Preston, Hopewell, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 2, 1945, Serial No. 571,033

The present invention relates to sound reproduction and more particularly to an improvement, in mud speakers.

Many factors govern the performance of a direct radiator loud speaker. The conditions which are favorable for maximum efficiency at the high frequencies are not the same as those which obtain for maximum at the low frequencies. In order not to overload the system, and the resultant production of harmonics, it is necessary to use a relatively heavy cone at the low frequencies. At low frequencies, the mass reactance of the air load is usually comparable to the mass reactance of a moderately heavy cone, so that the loss in efiiciency incurred by the use of a heavy vibrating system is not serious. In the high frequency range the main obstacle to good efiiciency is the large mechanical reactance of the vibrating system. A number of two separately drivencone systems have been designed and built from time to time. In one type, a high frequency loud speaker is mounted in the same baffle with the low frequency unit. In another type, the small high frequency loud speaker is mounted in front of the low frequency cone. All existing systems exhibit phase distortion and anomalies in the directional pattern in the crossover range because the two sounds from the low and high frequency loud speakers do not issue from a common origin.

Some of the objects of. the present invention are: to provide a direct radiator loud speaker which eliminates the undesirable features that are present in all two speaker arrangements heretofore in use wherein the high frequency comes 1 Claim. (01. 179-109) from one point and the low frequency from another; to provide a loud speaker characterized by an absence of undersirable wave pattern characteristics; to provide a simple, inexpensive loud speakercapable of covering the entire audible frequency range with the sound, coming from one source; to provide a duplex speaker wherein the destructive interference between the high and low frequencies is eliminated; to provide a loud speaker wherein the high frequency range is extended without impairing the power h'andling capacity; to provide a duplex speaker wherein a'largecone is a virtual continuation of a small cone; to provide a loud speaker formed of two cones having essentially the same sound origin; to provide a duplex speaker wherein the high frequency cone is separately mounted with respect to the low frequency cone and comprises an assembly readily removable as a complete 2 separate unit; and to provide other improvements as will hereinafter appear.

In the accompanying drawings, Fig. 1 represents a front elevation of a duplex speaker embodying one form of the present invention; Fig. 2 represents a section on line 2--2 of Fig. 1; Fig. 3 represents a section similar to Fig. 2 but illustrating another form of the invention; Fig. 4 represents a section similar to Fig; 2 but illustrating a preferred form of the invention; Fig. 5 is a wiring diagram of a circuit which may be employed for energizing the two voice coils; and Fig. 6 is a response graph of the speaker of the present invention taken over a frequency range of 30 to 15,000 cycles.

Referring to Fig.2 of the drawings, one form of the loud speaker system of the present invention comprises two nested overlapping cones I0 and l I made of vibratile material, such as heavy paper or the like now well known in the part of direct radiator loud speakers. The cone or diaphragm I0 is relatively large for the purpose of reproducing frequencie in the lower audible range of between 40 and 1500 cycles, while the cone or diaphragm l l is relatively small in order to reproduce frequencies in the higher audible range of between 1500 and 15,000 cycles. Both cones I0 and H are of truncate shape to provide in each case an annular base portion l2 and I3, the former being joined to an annular cylindrical voice coil M and the latter being joined to an annular cylindrical voice coil 15, the two coils I l and 15 thereby respectively forming the driving sections for the two cones.

For producing the magnetic field, a suitable magnetic structure is provided comprising, in the present instance, a permanent magnet 16 having an annular pole l'l encircling the voice coil I4, and a center pole l8 coaxial of the two cone base portions and having an end within and encircled by the voice coil l4. This pole I8 is formed with a through bore 19 and has an annular pole piece '20 mounted to encircle the voice coil 15 to thereby serve as the outside magnetic path of the small speaker. Within the bore 19 is a second permanent magnet 2| attached to a base 22, of soft iron fitted transversely within the bore I9 and locating the magnet 21 coaxially with respect to the base of the cone I l. A sleeve 23, also preferably of soft iron, encircles the magnet, 2| and abuts the pole 20, thereby forming a complete assembly readily removable from the pole piece l8. 'Ihe magnet 2| terminates in a cylindrical head, which in assembled condition is located within and encircledby the voice coil [5. Thus, the complete magnet assembly comprises the magnet 16, pole l1, voice coil i4, center pole l8, annular pole 20, voice coil l5, and magnet 2|, all of said parts being concentrically arranged in substantially the same transverse plane and coaxial with respect to the cone bases. From this construction it will be seen-that the lines of magnetic flux for poles I! and [8 include voice coil l4, while the lines of magnetic flux for poles 20 and 2| include voice coil l5.

For mounting the cone Ill a spider or annular dishpan structure 24 is attached at its end of smaller diameter to the annular pole piece I1 and terminates at its larger diameter end in a ring 2-5, to which the outwardly turned flexible margin of the cone I is clamped by a ring 26 and screws 21. The marginal edge of the small end of the cone i0 is supported by a flexible member or compliance 28 stretched between the cone end and a part attached to the pole piece H.

For mounting the cone il, a spider or outwardly flared annular dishpan structureifiis attached to the pole 20 and carries a ring 30 to which-the outwardly turned flexible margin 3| of the cone ll isclamped. The marginal edge of the small end of the cone H is supported by a flexible member or compliance 32 from a ring 33 anchored to the annular pole piece 20. A compliance 34 may be interposed between the two cones and allows the cone ll to vibrate independently of the cone ill.

Referring to Fig. 3 of the drawings, a modified form is illustrated wherein those parts which are respectively common to like parts in Fig. 2 are given like reference numerals since the description thereof remains unchanged. In this form the center pole 35, which is common to the magnets l6 and 2|, is formed as a solid block supported coaxially of the magnet i6 and having its other end provided with a recess 36 in which the magnet 2| is located. This magnet 21 is attached to a base 3] detachably connected to the bottom of the recess by screws 38 or like removable fastenings. Thus, provision is made for separately detaching and removing the high frequency cone ii and its associated parts without disturbing the low frequency cone iii.

Referring to the form of the invention shown in Fig.4, the modification concerns the mounting of the high frequency cone ll, while that of the low frequency cone remains unchanged from the forms of Figs. 2 and 3, and therefore the parts thereof are respectively identified by the same reference numerals. In the form of Fig. 4 the center pole 40 of the magnet system is a solid block, such as soft iron supported coaxially with respect to the cone l0 and having an end recess 4|, which receives a cup-shaped member 42, the bottom of which supports a permanent magnet 43 of a length to enter and be encircled by the voice coil 44, which in turn is encircled by the annular pole 45.

For mounting the high frequency cone 46, a flanged ring 47 is attached to the end of the center pole 40and secures themarginal edge of the large diameter end of the cone, The marginal edge of the small diameter endor baseof the cone 4-6 is supported by a suitable oomphance 4-8 from the pole 45, in which position it is arranged to be actuated by the voice coil 44. It should be noted that the length of the ring 4] (axially considered) is such that the slope of cone 46 is substantially the same as that of the cone ill, and the mounting so positioned reia tive tothe mounting of the cone .fll that the sur-' face of the cone 46 is substantially a continuation of the surface of the cone l0, and interrupted only to properl locate the voice coil 14. The leads 50 to the voice coils are brought into operative connection in any convenient way and terminate exterior of the speaker for cable attachment as will be understood.

In order that the high frequency speaker can be readily removed as a unit for repair or replacement, an elongated screw 5| passes axially through the pole 40 to thread into the base of the member 42 and thus retain the magnet 43 and the associated parts in place. When it is desired to remove the high frequency speaker, the voice coil leads 50 are disconnected from the exterior connections and the screw 5! removed so that the member 42 is free to be pushed out of the recess 41 by any suitable push rod entering the pole 40 through the hole vacated by the screw 5|, and of a length to shift the speaker unit outwardly to a position where it can be grasped and removed.

The pairs of voice coils used in conjunction with the respective forms of the invention may be connected as shown in Fig. 5 with a capacitance 52 of suitable value connected in series with the high frequency voice coil.

From the foregoingit will be seenthat by 10- cating the high frequency cone well back within the low frequency cone, it is possible to use the center pole ll of the larger cone as the outside magnetic path of the smaller cone. Thus, the large cone is a virtual continuation of the small cone because the cone angles are substan tially the same and the high frequency cone lies sufficiently within the cone H) to bring the two cone faces into substantial alinement. This alinementis brought about by so positioning the cone Ii within the cone 10 as to locate the large marinal edge of the cone ll substantially in the plane of the marginal edge of the small end of the cone l0. Since the smaller cone H has its own center pole in the magnet 22 it will be apparent that each cone is independently energized. By using two cones with essentially the same origin it is possible to use a simple overlap network, such as shown in Fig. 5 without encountering a ragged response in the overlap region. By reference to the frequency graph of Fig. 6, it will be seen, due to novel combination of a separately driven small speaker built into the center pole of the large speaker. that a smooth response :2 db. from 40 to 15,000 cycles is obtained. One of theoutstanding features of the speaker of the present invention is the uniform response, and smooth direction pattern in the overlap frequency range.

While the ma net illustrative of the preferred form of the invention is of permanent magnet construction it could be energized electro-magnetically, as will be understood. The high frequency response could be further extended by a. variation in the construction of the high frequency cone. The model described above has a flat frequency response, :2 db. from 40 to 12,000 cycles. The high frequency efiiciency of a direct radiator loud speaker is limited by the mass of the vibrating system. The high frequency response may be increased by the inclusion of a separately driven small speaker, built into the center pole of the large speaker which results in a smooth response, i2 db. from 40 to 12,000 cycles. One of the outstanding features of this loud speaker is the uniform response and smooth directional pattern in the overlap frequency range.

Furthermore, it should be particularly noted that the high frequency cone or diaphragm assembly is entirely independent of the low frequency cone or diaphragm assembly and hence the vibrations of neither cone can be transmitted to the other to produce the heretofore objectionable interference and distortion of the combined vibrations.

It is to be understood that in the foregoing description the term cone is used in a broad sense and as used in the specification and claim contemplates any form of vibratile diaphragm as an equivalent structure and within the scope of the invention.

I claim as my invention:

A loud speaker comprising a magnet forming an annular pole piece and a center pole piece of opposite polarity having an end bore, said pieces being spaced to form an annular air gap, a voice coil in said air gap, a second magnet of opposite polarity from said center pole piece in said bore, means accessible from the exterior REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,907,723 Bostwick May 9, 1933 1,908,957 Parry Nov. 3, 1934 2,122,587 Ringel July 5, 1938 

